Graft polymerization per se has long been known in the art with many graft copolymers such as ABS (acrylonitrile butadiene/styrene) resins achieving considerable commercial success.
It has also been known in the art that various vinylic monomers can be graft polymerized onto polymer substrates which have been first treated with ionizing radiation in the presence of oxygen or with ozone to form peroxy groups on the surface of said substrate. U.S. Pat. Nos. 3,008,920 and 3,070,573 teach the grafting of selected monomers onto ozonated polymer substrates.
While such a process would in theory seem to be a panacean method to modify at will the surface characteristics of any polymer substrate, such is not the case as is seen in the teachings of U.S. Pat. Nos. 4,311,573 and 4,589,964.
The objective of such graft polymerization is to modify the surface of the polymer substrate without causing major changes in the physical characteristics of the substrate as a whole.
Problems have arisen when such a graft polymerization process is carried out. One serious complication involves graft polymerization of the vinylic monomer onto the substrate as desired, but with the simultaneous and undesired homopolymerization of the vinylic monomer. This problem can be minimized by carrying out the graft polymerization process in the presence of a metal redox system using a variable valence metal ion in the reduced state to convert any hydroxyl free radical present to hydroxyl ion and thus minimize the simultaneous homopolymerization problem. See U.S. Pat. Nos. 3,008,920, 4,311,573 and 4,589,964.
U.S. Pat. Nos. 4,311,573 and 4,589,964 teach that another problem encountered in the surface grafting of a preformed polymeric substrate concerns depth and density control of the graft. If the bulk properties of the substrate are to be retained, then the graft depth should be no greater than necessary to modify the surface characteristics of the article. Grafts of excessive depth, grafts of insufficient density to achieve the desired property modification and the swelling and degradation of the substrate article during the process are serious problems plaguing this panacean process.
U.S. Pat. Nos. 4,311,573 and 4,589,964 teach a method aimed at inhibiting homopolymerization, at controlling graft depth and at accelerating graft polymerization to increase density, namely by carrying out the graft polymerization in the presence of a variable metal ion (ferrous) and a complexing agent (squaric acid) to control mobility of said ions.